Resin compositions comprising polyphenylene ether resins and polyamide resins have excellent heat resistance, forming processability, oil resistance, and organic solvent resistance. Resin compositions of this type which also contain elastomeric substances show excellent impact resistance. As a result, they have been used in automobile parts, electric and electronic parts, machine parts, etc., and are promising materials for use in various fields. However, because they contain polyamide resins, they have the drawback of dimensional change and lowering of elastic modulus due to moisture absorption. This is a particularly serious problem in applications such as automobile exterior body panels, which require high stiffness, dimensional stability, and impact resistance.
In order to increase their dimensional stability and stiffness, thermoplastic resins are often compounded with powders or flakes of inorganic fillers such as calcium carbonate, talc, mica, etc., or fibrous inorganic fillers such as glass fibers, carbon fibers, potassium titanate whiskers, etc.
It is possible to improve the dimensional stability of resin compositions by incorporating fillers such as calcium carbonate, mica, etc. for less mold shrinkage and lower linear coefficient of expansion. However, the increase in stiffness obtained in this way is inadequate, and the resulting material is more brittle, with lower impact strength. Glass fibers have poor alkali resistance, and so there are limitations on the applications of moldings containing them. Carbon fibers are easily obtained as long fibers, and show good chemical resistance, but they are expensive, and not very economical. Resin compositions incorporating these sorts of fibrous reinforcing agents also have other drawbacks, such as showing substantial anisotropy, having rough surfaces with low gloss, and causing severe wear to processing equipment and molds. Potassium titanate fibers, a more recent development, are easily obtained in the form of relatively long fibers, and show good chemical resistance, but because they are easily broken they cannot provide the reinforcing effect one would expect from their nominal fiber length. At times there is also some impairment of the plastic properties because potassium is present as a component. Potassium titanate fibers are sometimes used to impart color to the plastic as well, but because their coloring power is low they inevitably have to be added in large amounts, and this can actually result in lower strength, besides being uneconomical.
In order to overcome these drawbacks, needle-shaped titanium oxide has been used to fill thermoplastic resins, as described in Japanese Early Patent Disclosure Publication No. 1-103649. However, because those are very fine inorganic fibers with high aspect ratios, they easily become entangled with each other. The entanglement is not adequately broken when they are incorporated into resins, and as a result, products molded from those compositions tend to have fine bumps and depressions on their surfaces. In addition, the surface activity of the needle-shaped material is higher than that of granular titanium dioxide, and this can lead to molecular weight reduction in resins such as polycarbonates or polyamides. Consequently, these fillers cannot be used without surface treatment. When they are surface treated with silane coupling agents or titanate coupling agents as described in Japanese Early Patent Disclosure Publication No. 1-103649, their reinforcing effect increases, but there are also the problems that the melt flow of the resulting resin compositions is impaired, and the tendency toward surface roughening is even greater since it is difficult to disentangle the fibers.